Using Mount / UnMount Commands in Shell Scrips

[udayabaskaran]

My intention is to write a script/exe which will mount and unmount optical disks.
This script has to be executed by a non root user. The script is owned by root and i have given suid. When this executes i see the disk getting mounted. But during umount it says "permision denied for /etc/mnttab".
when i do a "bdf" i can see the disk mounted and entry is there in "/etc/mnttab" also.

I have created a c program and gave the same permissions and tested. still i get the same error.

What's wrong with the approach? How to get the umount to work. The effective user id is root in both scenarios.

[A. Clay Stephenson]

Hi:

You need to be very clear here. In your C program, are you using the umount() system call or are you simply do something like
system("umount filesys1"); I would use the system call itself and if it returns a non-zero value then examine errno. It is very important to know what errno is set to. My best guess is that for some reason your setuid is not working as you expect.

It would also help to know what OS you are running and the type of filesystem. Are you using pfs_mount/pfs_umount? Are you running OmniStorage?

[Olav Baadsvik · Accepted solution]

Hi,

Here is a couple of programs I wrote some time
ago after getting tired of doing su every
time I wanted to mount/unmount a cd.
I would think you can use this for your
purpose. As you see the device-file and
mount-point is hardcoded. You can easily
rewrite the programs to take these as
arguments.

/* Program that gives a regular user the ability to mount a cd.
. Compile the program
. Make root the owner of the executable.
. chmod 4755 executable
Olav Baadsvik Oslo Norway.
*/
#include
#include
#include
#include

main ()
{
int ios;
struct cdfs_args cda;
char buf[128];

setuid(geteuid());
setgid(getegid());

cda.fspec="/dev/dsk/cdrom";

ios=sysfs(GETFSIND, MNTTYPE_CDFS);
/* printf ("sysfs for %s er 0x%x\n", cda.fspec, ios); */

sysfs(GETFSTYP, ios, buf);
/* printf ("sysfs fstyp er >%s<\n", buf); */

ios = mount ( cda.fspec, "/CDROM",
( MS_DATA|MS_RDONLY),
MNTTYPE_CDFS,&cda, sizeof(cda) );

/* printf ("ios=%d\n", ios); */
if (ios) perror ("cdmount");
}

Here is the unmout-code:
#include
#include
#include

main ()

{
setuid(geteuid());
setgid(getegid());
umount("/CDROM");
}


Regards
Olav

[Robin Wakefield]

Hi,

I thought mount ignores the effective user id, and *always* uses the real user id to determine privileges.

Rgds, Robin.

[Ravi_8]

Hi,
umount doesn't use suid, it uses effective uid

[A. Clay Stephenson]

Hi:

I just wrote a 3-minute setuid C program that mounted and unmounted a filesystem with absolutely no problem as a regular user.

I also used my dangerous version of a setuid wrapper which does a setuid and then can exec a command and it worked as well.

e.g.

cemexec root /usr/sbin/umount /filesys1
cemexec root /usr/sbin/mount /filesys1

Both of these worked exactly as expected.

[Douglas Arneson]

Hi,

We use a simple script that is owned by root:sys and executable by all with access to the shell. The scripts even run by cron.

The cdoff script:
umount /dev/dsk/c0t2d0
unount /dev/dsk/c0t2d1
umount /dev/dsk/c0t2d2

The cdon script is just the opposite of cdoff. mount instead of umount.

Simple, works for us as the users have to change CDs once a month as new CDs are distributed.


HTH,

Doug

[Heiner E. Lennackers]

Hi,

maybe you should consider of using sudo, downloadable at http://hpux.cs.utah.edu/

Heiner

[Ken Stallings]

I agree with the sudo approach. You also get the ability to specify exactly what options are allowed to the command for better security.

[Ulrich Deiters]

The attached script enables me to mout or unmount various exchangeable media. It uses ssh to control the access; users wanting to use the script must have their public ssh key entered into root's access control file.

[Bruno Dostie]

create file name:
vi /usr/bin/mountcd

#!/usr/bin/sh

/usr/sbin/mount /dev/dsk/... /mountdir


save file,

chown root /usr/bin/mountcd
chgrp sys /usr/bin/mountcd

chmod 4555 /usr/bin/mountcd

" root user to execute file"

Regards

Bruno